<script>
alert("Executing script in the child window.");
var x = new XMLHttpRequest();
x.open("POST", "http://example.test:8000/xmlhttprequest/resources/access-control-allow-lists.php?origin=*&headers=Content-Type", false);
alert("XHR POST opened.");
try {
    x.send(1);
    alert("XHR POST sent.");
} catch (e) {
  alert("exception! " + e);
}
if (window.opener)
    window.opener.postMessage('done', '*');
</script>
